multipass is recommended by the Ubuntu website.

It is extremely hard to locate the disk image used by multipass. It turns out to sit in the directory included in the command below. Besides that, it has been a pleasing experience using this tool.

Use this command to resize the disk image on macOS.

VBoxManage modifyhd --resize 15000 /var/root/Library/Application\ Support/multipassd/virtualbox/vault/instances/ubuntu-lts-multipass/ubuntu-20.04-server-cloudimg-amd64.vdi